1. Filed of the Invention
The present invention relates to an image forming apparatus capable of operating to form an image both in a horizontal installation and in a vertical installation of the apparatus.
2. Description of the Related Art
In an image forming apparatus such as a copying machine, a printer, a facsimile, etc., a latent image forming apparatus like an electrophotographic apparatus has been utilized from a demand for recording on ordinary paper. In such an image forming apparatus, an electrostatic latent image is formed on a photosensitive drum. Next, the electrostatic latent image on this photosensitive drum is developed with a powder developer and is thereby transformed into a visible image. Further, the powder developer on the photosensitive drum is transferred onto a sheet. This sheet is separated therefrom, and the powder-developed image on the sheet is fixed.
In this type of image forming apparatus, it is desirable to make effective use of an installation space by changing an installation position of the apparatus.
FIG. 23 is a view of assistance in explaining a prior art.
For example, Japanese Patent Laid-Open Publication No. 4-323125 discloses an image forming apparatus capable of operating even when the apparatus is installed horizontally or when installed vertically. In this image forming apparatus, the sheet is fed upward in the vertical installation of the apparatus, and, therefore, a fixing unit is positioned above a photosensitive drum. Then, a developing unit is positioned under the photosensitive drum. Accordingly, in the horizontal installation of the apparatus, the developing unit supplies toners in the horizontal direction, and hence the toner supplying direction becomes an antigravity direction when installed vertically.
On the other hand, as illustrated in FIG. 23, a thermal fixing unit 91 includes a heat roller 91-1 and a backup roller 91-2. A frame 91-3 is provided along the periphery of this heat roller 91-1. This frame 91-3 covers the periphery of the heat roller 91-1, thus making an attempt to improve a thermal fixing factor.
In this thermal fixing unit 91, a water vapor is generated by heating up the sheet. This water vapor is diffused together with the heat within the apparatus. As depicted in FIG. 23, some heat and some water vapor run through between sheet guides 92, 93 and the frame 91-3 and are diffused within the apparatus. When this water vapor is stuck to the photosensitive drum 90, an adhesion of the toners is produced, resulting in a deterioration in terms of a printing result. Further, when the heat is given to the photosensitive drum 90, a photosensitive layer of the photosensitive drum 90 is also deteriorated.
Under such circumstances, a known apparatus among the image forming apparatuses installed only in the horizontal position is provided with an exhaust duct (e.g., Japanese Patent Publication No. 1-105277).
In this apparatus capable of being installed horizontally and vertically, it is required that a developing agent be fed upward when installed vertically according to a conventional construction in which the developing unit is positioned under the photosensitive drum when installed vertically. For this reason, the developing action is easy to become unstable, and, besides, a large feed mechanism is needed. This construction is therefore unsuitable for a small-sized apparatus.
In contrast with this, it is desirable to take such a placement that the developing unit is positioned above the photosensitive drum when installed vertically. With this placement, however, the fixing unit is disposed under the photosensitive drum, and it follows that the heat and the water vapor in the fixing unit rise directly toward the photosensitive drum. This brings about a decline in terms of an image forming function of the photosensitive drum as well as a deterioration of the photosensitive drum.
Further, as stated above, the direction of a release air flow from the fixing unit varies depending on the horizontal installation and the vertical installation. It is therefore difficult to smoothly control a fixing temperature of the thermal fixing unit.